Burner safety control



1946. D. E. RICHARDSON ETAL 2,410,524

BURNER SAFETY CONTROL Filed June 28, 1941 6 Sheets-Shoot 1 whiz f h 26 L.Lg V

S ZIPPL Y 51 4, J- a91L f Z' 37 I L, 4/ 42 l WMbI/PS.

Mel ll: aes (7.31 'Mztlf fimaaww 6 Sheets-Sheet 2 Nov. 5, 1946. D. ERICHARDSON ET AL BURNER SAFETY CONTROL Fild June 28, 1941 worrr gai 5,1946. D. E. RICHARDSON ETAL 2,410,524

BURNER SAFETY CONTROL Filed June 28, 1941. 6 Sheets-Sheet 3 110 1/- waslPt'cizardsvz/z M NW 4 D. E. RICHARDSON ET AL 2,410,524

BURNER SAFETY CONTROL Fil ed June 28, 1941 s Sheets-Sheet 4 Nov. 5,1946.

D. E. RICHARDSON ET AL I BURNER SAFETY CONTROL Filed June 28, 1941 6Sheets-Sheet 5 Nov. 5, 1946. D. E. RICHARDSON ET AL 2,410,524

BURNER SAFETY CONTROL Filed June 28, 1941 a Sheets-Slieet e PatentedNov. 5, i946 BURNER SAFETY CONTROL M Donald E. Richardson and Robert E.Yates, cm-

111., assignors to. Drying Systems, Inc.,

' Chicago, 111., a corporation of Illinois Application June 28, 1941,Serial No. 400,332

I 16 Claims.

This invention relates to fuel burners and like heating devices andparticularly to safety con- ,trol apparatus for use with such devices.

Heating, baking and drying systems embodying fuel burners or the likeare subject-to hazards that arise when fuel introduced into a burnerisnot properly ignited and is thereby enabled to accumulate in anunburned condition in the system wherein the productsoi combustion fromthe burner'are normally utilized, thereby create ing a danger ofexplosion'or other serious damage. It is therefore essential thatsystemsor installations of the aforesaid character be equipped withmeans for detecting failure of combus ion and which will respondautomatically in such a contingency to eflect a control operation, suchas. shutting ed the supply'of' fuel to i the burner or burners, and thussafeguard the system against risk of. explosion or the like Numeroustypes of safety control means or apparatus of the aforesaid characterhave been heretofore proposed. The earlier forms of such apparatusentailed the .use of heat-sensitive devices such "asthermostats.'thermocouples, pres-- ,sure' diaphrasms and the like whichwere 'indirectly responsive to thepresence or absence of a flame in aburner for eifeotingwhatever con- .troi operations were necessary-withrespect'to the burner. Such devices were generally unreliable because anappreciable time lag was en- .tailed in the operation thereof and suchdevices were readily affected by disturbances such as external airdraitsflwhiohhad no direct relation Later 1 forms of flame detectorshave operated in deto combustion conditions in the burner.

pendence ;upon' the flow of electrical current across a gap-between apair of spaced electrodes I that are arranged to be bridged by the flameof the burner. Usually one of these electrodes constitutes what is knownin the art as a flame rod which is disposed in the flame zone of theburner so as to be contacted by a flame when one is present, while theother electrode may consist of another time rod similarly situated or itmay be afforded by a portion of the burner structure itself. Flamedetectors of this character roughly divided into several classes. a

may be Oneclass'of such apparatus entails anarrangement in which thevalue ofan ohmic resistanc of the electrical path betweenfthe electrodesis the determining factor in the operation of the apparatus, themagnitude of this resistance being governed by the presence or absenceof flame in the burner. This class in turn may be said to comprise atleast two varieties. In one of these theelectricalcurrent flow throughthe flame path is Produced by impressing a relatively high voltageacross the flame-sensing electrodes,

this'voltage being suflicient to set up a current 5 having a greatenough magnitude-(provided a flame is present) to operate a relay Or thelike. In the other form ofthis apparatus an alterhating-current voltageis impressed upon the flame-sensing circuit and an electron tube meansis arranged to b governed not only according to the magnitude of thecurrent flow, through the flame gap,- but also in response to the phaserelation of this current with respect to the impressedfvoltage. Anexample of the last-mene tioned apparatus is disclosed in the,copendin'g application of Robert Yates; Serial No. 245,-

173, flied December 12, 1938, now Patent No. 2,282,551,- patented May12, 1942, and one of its outstandingadvantages is that it operates todetect not only flame failure but also an unsafe condition which mayarise when the flame gap is --accidentally. short-circuited. I

Another-of the classes of flame detectors to whioh'reierenoe hasbeenmade hereinabove entails anarrangement in which the conduction ofelectrical current through the flame is made to depend not so much uponthe conductivity of the flame itself as upon the electron-emittingproperties of an electrode which is arranged tobe 3 heated by the flame.Buch'flame detectors tend to be unduly sensitiveand moreoverare,unsatisfaotorywhen used on alternating-current because of the factthat the flame itself has a current-rectifying effect which tends toblock the a unidirectional now of current set up by thermionic -emissiOn1m1ess the electrodes are so arranged that the emitter electrons travelfrom the hot-to the cool electrode substantially in .direction of flamepropagation (that is, away 40 from the burner nozzle). It isdisadvantageous 1 to arrange theelectrodes in this manner, how ever,because-the cool electrode must then be placed outside of, but veryclose to, the edge of the flame and any fluctuations of the flame due tocross-drafts .or the like may produce corresponding fluctuations inthe'flow of electron current, with detrimental effects upon the accuracyof operation.

Still another classof flame detecting safety devices is that wherein thearrangement makes use of the substantially unilateral conductivity of aflame without relying upon any thermionic emission characteristics of anelectrode. Be-

cause of the greater ease with which a current pl; of electrons ornegative ions may flow in the direction of flame propagation thancontrary thereto, a substantially unidirectional flow of negative ionsor electron current takes place between the spaced electrodes in adirection away from the burner nozzle when an alternating-currentvoltage is impressed across these electrodes. This unidirectional orrectified current is utilized to charge a dielectric body such as acondenser to thereby control the potential applied to the controlelement or grid of an electron tube and thus regulate the amount ofcurrent flow in an output circuit of the tube. If the flame isextinguished or becomes too low, the rectifled current flow ceases andthe condenser loses its charge, the effect of this being to so conditionthe electron tube circuit that a control operation is effected or awarning signal given.

The present invention falls in the last mentioned class, but it affordsseveral features which are not realized in prior devices of thischaracter, and particularly it takes into consideration the possibilitythat the flame-sensing means itself. may become defective in severalrespects and thereby cease to afford protection for the burner system.For example, a flame-sensing rod or electrode may become softened due tothe heat of the flame and may therefore so bend that it engages a partof the burner. This is dangerous because the flame is short-circuitedand is therefore no longer included in the input circuit of the electrontube, so that the apparatus furnishes a false indication of thepresenceof a flame unless it is adapted to function safely upon the occurrenceof such a contingency. Then again, instead of there being a directelectrical contact of the flame rod with the burner there may be.

an accumulation of hygroscopic, carbonaceous or. other electricallyconductive substance extending between the flame rod and the burnerwhich has a leakage or shunting effect that may simulate a flame evenwhen there is no combustion. of the fuel in the burner. It is likewisepossible that suflicient leakage may take place through the insulationsurrounding the conductors in the flame-sensing circuit to produce acondition simulating the presence of flame in the burner. Prior safetycontrol devices operating on the flame rectification principle wereunable to prevent misoperation of the system under at least some of thecircumstances enumerated above and therefore an important object of thepresent invention is to improve such safety apparatus in these respects.

. Heretofore it has been proposed to afford means for checking leakageeffects such as have just been described in a number of ways, as byproviding one of the flame-sensing electrodes with a sleeve that isintended to intercept any leakage current passing between the electrodesby apath other than through the burner flame, and upon the occurrence ofexcess leakage from an electrode to such a sleeve the apparatus isintended to react as though there had been a flame failure. Such anarrangement is unsatisfactory because obviously the sleeve cannot extendinto the flame none since it would then prevent the proper operatlon ofthe device, and hence it is ineffective to detect leakage paths in thisregion.

Other *prior devices of the character which function in dependence uponthe ohmic resistance of the flame path have dispensed with the specialsleeve on the flame-sensingelectrode, and it has been intended incertairf nf .these devices that they are to react safely whenever thereis any the normal flow of current through the flame, whether this existsby reason of short-circuiting, insulation failure or carbonization ofthe electrode in the region Of the flame itself. In these prior devicesan all-around leakage check of the flamesensing circuit is made as acondition precedent to operation of the system. However, once theoperation of the burners has been initiated, the

safety apparatus thereafter functions to detect only flame failure orthe existence of a lowresistance shunt pathbetween the flame electrodes,and it will not operate safely upon flame failure if a leakage pathhaving a resistance comparable with that of a normal flame path isestablished between the flame electrodes in the region of the flamewhile the burner is in operation. Usually in such burner systems a safereaction of the control apparatus in response to the detection ofcurrent leakage conditions in a burner does not occur until the systemhas been recycled, that is, disabled due to the operation of some othermeans such as a room thermostat and then started up again, and if suchrecycling does not occur soon enough, a risk of explosion is created.

' Hence, a further object of our invention is to enable the controlapparatus to recognize all of the unsafe conditions enumeratedhereinabove. and particularly to ascertain whether the electrlcal pathexisting between the flame-sensing electrodes while the burner is inoperation is afforded by the flame itself or whether the electrodes havebeen electrically connected in some other manner, especially by abilaterally conductive shunt or leakage path having an ohmic resistanceof a value comparable with that of a normal flame path.

In flame-detecting devices which embody electron tubes, particularlythose of the gas-fllled type, it is desirable to have the tube performas little work as possible so as to prolong the llie of the tube.Normally the control apparatus may well be in an idle condition andneedbe called upon to function only on comparatively rare occasions, asin the event of flame failure or upon the occurrence of some otherhazardous condition against which the apparatus is designed to guard,and even then it need operate for only a short time. Hence, it isadvantageous to so arrange the tubes that they remain nonconductivewhile the burner system is in normal operation, or in other words duringthe greater part of the time, and become conductive only in an emergencyrequiring operation of the control apparatus, and so to do is an objectof a preferred form of the present invention.

Electronic safety control apparatus of the character in which anelectron tube is adapted to be rendered nonconductive when the presenceof a flame in the burner is sensed may become subsubstantial leakagefrom an electrode other theta.

Ject to misoperation due to internal failure of the tube. For example,the tube may become nonconductive due to burning out of the filement orbecause of other circumstances tending to decrease its-conductivity sothat the tube tends to give a false indication that a flame is presentin the burner even though this may not actually be the case. In theaforesaid copending application, Serial No. 245,173, there is disclosedmeans for causing the apparatus to react safely in the event one or moreelectron tubes become defective due to filament failure. However,inasmuch as this is only one of the possible causes of tube failure, wehave in the present instance provided means forautomatically checkingeaehelectron tube to determine whether or system;

not it is capable of operating properly and to detect failure of thetube due to any of a number of causes. Preferably such checking of thetubes should take place prior to the initiation of operation of theheater system, and so to do constitutes another object of the invention.

A still further object is to adapt the safety control apparatus for usein multiple burner systems in a novel manner without unnecessaryduplication of parts and at the same time to afiord the features andadvantages described hereinabove.

A specific object of a preferred form of the present invention is to soarrange an electron tube and a condenser in relation to an alternatingcurrent source and to the flame-sensing electrodes that thesubstantially unilateral conductivity of the electrical path through theflame normally enables the condenser to become unidirectionally chargedduring those half-cycles of the alternating current in which the tubeisinoperative (that is, in the inverse half-cycles).

An ancillary object is to so control the tube by the condenser that thetube remains continuously nonconductive if and.when a. substantialunidirectional charge exists on the condenser, whereby, if this chargeis dissipated due to the establishment of a bilateral electricalconnection between the flame-sensing electrodes, such as may be causedby a short circuit ora. leakage shunt, or if this charge leaks of!without being replenished because of flame failure, the tube becomeseffective to conduct current during forward half-cycles of thealternating current.

Other and further objects of the present invention will be apparent fromthe following description and claims and will be understood by referenceto the accompanying drawings which, by way of illustration, showpreferred embodiments and the principle thereof and what we now considerto be the best mode in which we have contemplated applying thatprinciple. th-

Fig. 2 is a diagrammatic view similar to Fig. 1

but illustrating one manner in which the invention may be adapted foruse in a, multiple burner system; i

Fig. 3 is a. schematic view of a master control unit embodyingvarlousimprovements over the apparatus shown in Fig. 1;.

Fig. 4 is a. schematic view similar to Fig. 3 and showing another mannerin which the apparatus may be incorporated in a multiple burner Fig. 5is a simplified wiring diagram of the apparatus illustrated in Figs. 3and 4;

Fig. 6 is a diagrammatic view showing the manner in which spare controlunits may be provided;

Fig. '7 is a plan detail view of a tube socket as it is mounted in acontrol unit;

Fig. 7A is a sectional detail view taken sub stantially on the lineIA-IA on Fig. '7;

Fig. 7B is a partly sectional detail view of a screw shown in Fig. 7A;and

Fig. 8 isa diagrammatic view of a modified form of the invention.

Referring first to Fig. 1, wherein the basic construction is shown, itwill be observed that the apparatus is adapted to control the operationof a burner generally designated I2 which is supplied with fuel throughpipe I3 that is grounded as indicated at H. A self-closing fuel valve I5having an operating solenoid i6 is embodied in the fuel supply pipe iito prevent the supply of fllel to the burner l2 when the solenoid i6 isin a deenergized condition. The burner I2 is equipped with aflame-sensing rod or electrode II which is insulatedly mounted in such amanner as to extend into the flame F of the bumer when such a flame ispresent, the electrode I! being maintained in predetermined spacedrelation with the tip or nozzle of the burner l2.

as H positioned closer to the nozzle of the burner than the first rod i!may be employed equally well for this purpose.

The control apparatus constituting the preferred form of the presentinvention includes an electron tube l8 which is diagrammaticallyrepresented in Fig. 1 as being of the tetrode type, having a shield grid65 connected directly to the cathode l9. Other types of tubes such astriodes may be utilized in place of the tube 18, but for the purposes ofthe present invention a tetrode has been found to be the mostsatisfactory. We prefer to employ a gas-filled electron tube III inorder to afford a trigger action, but here again it is to be understoodthat we do not limit ourselves to such a tube since a vacuum tube may beutilized in lieu thereof if desired. The cathode l9 ofthe tube I8 iselectrically-connected to a line wire Li which leads to. one pole of asuitable source of alternating current. A line wire L2 leading from theother pole of said source of current is electrically connected to oneend of'the winding of a relay 20, the other end of this relay windingbeing connected by a conductor 2| to the anode 22 of the tube ii. Theheater orfilament 23 of the tube I8 is electrically connected to asuitable filament voltage supply source,

The relay 2|! has a contact 24 which is normally closed when the relayis in a deenergized condition so as to establish an electricalconnection from a conductor 25, which leads from one end of the windingof the fuel valve solenoid IE, to another conductor 26 that leads to onepole of a suitable source of voltage. The other pole of this source ofvoltage is electrically connected by a conductor 21 to the other end ofthe winding of the solenoid i6. Thus the solenoid i6 is maintainedenergized so long as the relay contact 24 is closed, and while it is inthis condition it holds the fuel valve ii open to enable the supply offuel to the burner l2. However, if the relay contact 24 opens uponenergization of the relay 20,

the solenoid I6 is deenergized and the fuel valve 7 tive under normalconditions when there is combustion of the fuel in the burner, so thatthe relay 20 remains deenergized and thereby enables the fuel valvesolenoid it to remain operative for maintaining the fuel valve l open.

Since the tube I8 is to be responsive to combustion conditions in theburner l2, means have been provided to control the operation of thistube in accordance with the presence or absence of a flame as F in theburner, Thus, the control element or grid 28 of the tube It is connectedthrough the medium of a protective resistor 29 to a conductor 30 whichis electrically connected to the flame-sensing electrode l1. Thisconductor 30 is also electrically connected by a resistor Ii to aconductor II which is connected by the filament of a lamp 3! to the wire32 leading to the cathode i9 and line wire Ll, In order to prevent strayelectrical disturbances from affecting the tube II, the conductor 30,which in ordinary practice is a t to be of considerable length, isenclosed in a metallic sheath II which is insulated therefrom throughoutits length, this sheath 34 being electrically connected to the conductorI. Usually the dielectric insulating medium between the conductor 30 andthe sheath 8 aflords a slight amount of electrostatic capacity, whichhas been diagrammatically represented in Fig. 1 as being lumped in asingle capacitance 8|. hereinafter referred to as a condenser.

- In order to complete the flame-sensing circuit the line wire L2 isgrounded as indicated at It so that whenever a flame F is present in theburner I! an electrical circuit is established from line wire L2 throughthe burner l2, flame F, electrode I I, onductor Ill, and through theresistor II and condenser 35 in parallel to the conductor ll, thencethrough the lamp 33 to the cathode 8 F between the electrode I1 and theburner it. As the apparatus is arranged in Fig. 1, the tube It isconductive only during those half-cycles of the alternating current inwhich the line-wire L2 is positive with respect to the line wire Ll, orin other words during the forward half-cycles. Under these conditionsthe electrode I1 is negative with respect to the burner 12 and thereforethe flame F is effective to block the flow of electron current betweenthe burner l2 and electrode I1. 'During the inverse half-cycles, inwhich the polarities of the line wires Li and L2 are reversed, the tubei8 is nonconductive but the flame F then conducts current. Under thelatter circumstances, inasmuch as the line wire Ll is at positivepotential with respect to the line wire LI, the condensers "and 31 areso charged that the grid 28 of the tube I8 assumes a negative potentialwith respect to the cathode la. The full charge on these condensers maybe built up throughout a series of inverse half-cycles, the length oftime required to impart a substantially steady charge to thesecondensers being determined by the values of capacity and resistance inthe circuit. Once the condensers have assumed their full charge, thegrid 28 is imparted sufficient negative bias to prevent conduction ofcurrent through the tube l8. The resistor Ii return wire 32 which iselectrically connected to the line wire Ll. To augment the path affordedby the condenser 35 and resistor 8!, another condenser 81 may beelectrically connected across the conductors 30 and 4| in parallel withthe resistor ll. Ordinarily the amount of current has a high enoughresistance so that the condenser charge is not appreciably diminishedduring the forward half-cycles so long as a flame P is maintained in theburner l2. Under these conditions the relay 20 remains in a deenersizedstate to thereby maintain an energizing circuit to the fuel valvesolenoid l6, and hence fuel continues to be supplied to the burner l2.

If for some reason the flame F should become extinguished or unduly low,however, the above described flame-sensing circuit is broken and thecondensers l5 and I1 discharge through the resistor 3|, causing the grid28 to lose its negative bias, and thereupon the tube is is renderedconductive during forward half-cycles of the alternating current. Hence,a pulsating rectiflowing through this flame-sensing circuit is so fledcurrent passes through the winding of the minute as to be incapable oflighting the lamp ll, except when certain kinds of short-circuits occur,as will be explained.

For the purpose of further protecting the derelay 20, causing this relayto energize and open its contact 24. Preferably the relay" is of theslow-to-release type so that it maintains its contact 2| open so long aspulsating current is device from stray electrical disturbances as wellas livered to this relay winding by the tube ll.

to prevent accidental injury by electrical shock to persons working withthe apparatus, the cable comprising the conductor 30 andsheath 84 may bepassed through a conduit 38 which is grounded The energizing circuit forthe fuel valve solenoid It is thereby broken; causing the fuel valve IIto close and prevent any further supply of fuel to the burner it.

as indicated at 38, the sheath ll being suitably 58 For the purpose ofinitiating operation of the insulated from this conduit. The presence ofthe sheath 3| intermediate the conductor I0 and grounded conduit 38 hasa tendency to prevent excessive leakage through the insulationsurburnerafter it has been disabled by the safety apparatus a normallyopen starting switch ll may be connected in parallel with the relaycontact 24 in the fuel valve energizing circuit. This rounding theconductor 10, inasmuch as this 00 switch is momentarily closed 'by theoperator to sheath lslmaintained at substantially the same potential asthe line wire Li and therefore tends to divert any leakage currentproceeding from the line wire L2 by way of ground away from thecondoctor I0.

It has been explained herelnabove that a flame energize the solenoid l6independently of the 'relay 20 to enable fuel to be supplied to theburner II. The fuel is ignited by suitable ignition means (not shown)which is temporarilyrendered operative for the purpose of starting theburner in operation, such ignition means preferably being under thecontrol of the starting switch 40. When the operatorascertains that thefuel has been properly ignited he releases the switch 40 and thereafterthe valve energizing circuit is maintained closed through the contact 24of the. relay Ill (the tube I! having been rendered nonconductive uponthe establishment of a no current flow can take place through the flame15 name F in the burner it) unless and until a hazardous condition againarises requiring a vshut-down of the system.

There are occasions in practice when the safety control apparatus isrendered partially defective by unwanted leakage or short-circuitconditions in the flame-sensing circuit. For example, the electrode llmay accidentally come into contact with the burner l2 or to some othergrounded part of the structure, or there may be a leakage path existingbetween the electrode I! and burner l2 due to accumulation of carbondeposits or I line wire L2 so that the tube I3 is rendered conductiveduring forward half-cycles. Similarly, if there is a leakage pathbetween the electrode i1 and burner |2,then because of thenon-rectifying or bilateral character of this leakage .path thecondensers as 35 and 31 do not assume any appreciable unidirectionalcharge. Any charge which the condenser receives during the inverse half-cycle is generally lost during the succeeding forward half-cycle sothat the grid 23 has no opportunity to retain its negative bias andtherefore the tube I8 is rendered conductive to interrupt operation ofthe burner I2.

It should be noted in this connection that safe operation of theapparatus is insured even though the leakage resistance in the flame gapis of a value approximately equal to the ohmic resistance of a flame asF, and this is true even though the false flame resistance comes intobeing while the burner is in operation. In most instances wherebilateral conduction through a leakage path occurs, the response of theapparatus is practically instantaneous. In other instances, where theleakage resistance is of a critical value, the apparatus may wait untilthere'is an actual flame failure (if and when this occurs) beforeoperating to shut off the fuel supply to the burner. In any event,however, the emission of unburned fuel is prevented, and it is notnecessary as in the case of prior types of control apparatus to gothrough the process of recycling the system (that is, effecting ashutdown by other means and then starting the system inoperation again)in order to so condition the apparatus that it will disable the fuelsupply means upon detecting current leakage in the flame path.

Another undesirable condition may exist when there is excessive leakageor a direct short-cir-' cuit between the conductor 30' and the sheath,such as may be caused by a failure of the insulation. In this case theamount of resistance in parallel with the condensers 35 and 31 isgreatly reduced so that these condensers discharge practicallyinstantaneously during forward 'halfcycles of the alternating current,thereby render ing the tube I8 conductive with the effect aforesaid.There is also a possibility that the sheath 34 may become grounded,causing the filament of the warning lamp 33 to be connected directlyacross the line wires LI and L2. This produces a relatively heavy flowof current through this filament, causing the lamp 33 to light. The tubel3 also becomes conductive due to the shunting of the condensers 35 and31. and the relay 20 is thereupon energized to shut off the fuel supply.

It has been stated hereinabove that the present invention may be readilyadapted for use in multiple burner systems, and one such arrangement forachieving this object is diagrammatically illustrated in Fig. 2.v Thus,there is provided a master control unit designated MU which issubstantially identical with the device shown in Fig. l, and which isoperatively associated with one of the fuel burners l2 included in abank of bumers that are supplied with fuel by a single pipe I3 havingthe necessary branch connections to the various burners. Correspondingparts of the structure shown in Figs. 1 and 2 are designated by likereference characters and it will be understood that the operation of themaster control unit MU is substantially the same as that of theapparatus shown in Fig. l.

In addition to the master control unit MU there are provided a pluralityof secondary control units designated SU which are respectivelyassociated with the other burners I2 01' the system in much the samemanner as the master unit MU Thus, each is associated with the burnerl2. burner I2 has an electrode I1 which is connected by a shieldedconductor 30" to the control grid 28' of the electron tube ill in therespective secondary unit SU. The shielded conductors 30 and 30' may bepassed through individua1 grounded conduits such as 38, Fig. 1, or itmay b preferred to run these conductors through a common Y conduit forat least a portion of their respective lengths. The secondary units aresubstantially identical in construction with the master unit MU exceptthat they do not include any relays.

the cathodes I! of the tubes III are all electrically connected by wires32 and 32', respectively, to a common conductor 46 leading to the linewire Ll. Hence, the electron tubes [8 and I8 in the various controlunits are all connected in parallel and the combined anode current ofthese tubes passes through the winding of the relay 20. The conductors30' are connected through the medium of resistors 30 and condensers 31'to conductors 4|, which are also connected to the sheaths 34', and fromthe conductors 4| electrical connections with the respective cathodereturn wires 32' are established through the fllaments of lamps 33embodied in the units SU.

It will be recalled that when a flame as F is present in a burner as I2the associated tube as I8 is rendered nonconductive at all times.Similarly, when the flames are present in all of the other burners l2the corresponding tubes I8 are all nonconductive, and under thesecircumstances the relay 20 remains deenergized. However, if

a flame in any one of the burners becomes ex- .tinguished, or too low,the tube associated with such burner is rendered conductive so thatcurrent flow takes place in the common anode circuit including thewindings of the relay 20, which relay thereupon energizes and opens itscontact i 24. The fuel supply pipe I 3 has a valve l5 controlled by thesolenoid I6 which receives its enersizing current through the relaycontact 24. but

when the relay 2!! is energized to open the contact 24 the solenoid 1-6deenergizes and enables the fuel valve It to close and thereby interruptthe supply of fuel to all of the burners in the system. One of theadvantages of arranging the electron tubes to be nonconductive undernormal combustion conditions is that the relay 2') need merely besensitive to any substantial current flow whatsoever in the common anodecircuit. On the other hand, if the tubes were arranged to be conductivewhen flames were present in their corresponding burners, andnonconductive in the absence of flame, the relay 20 would be required todetect a proportional decrease in total anode current for each tuberendered nonconductive. In a multiple burner system having, say, sixteenburners and associated flame-detecting tubes, the operation of the relay20 might become quite critical and on that account easily influenced byexternal factors such as fluctuating line voltages having no relation tothe combustion conditions in the burners. Hence, we prefer that theelectron tubes should be adapted to pass current only under abnormalconditions.

The other safety features described hereinabove in connection with thecircuit of Fig. 1 are also afforded in the case of each and everycontrol unit in the multiple system as shown in Fig. 2. Thus, if thereis a shunting of the flame gap in any particular burner due to groundingout of the flame-sensing electrode or the building up of anon-rectifying resistance path across the flame gap as a result ofcarbonization and the like, the electron tube in the control unitassociated with that burner is rendered conductive to shut off thesupply of fuel. The same is true if there is an insulation failure whichcauses excessive leakage in the cable including a conductor 30 or 30'leading to the flame electrode of a particular burner. Hence, adependable safety control apparatus for a multiple burner system isafforded with a minimum duplication of parts, it being noted that onlyone relay 20 and fuel valve l need be employed. The other portions ofthe apparatus such as electron tubes, resistors and condensers arerelatively inexpensive so that the construction is quite economical inaddition to being reliable.

It has been mentioned that another object of the present invention is toprovide means for automatically checking the conductivity of theelectron tube or tubes embodied in the safety control apparatus as acondition precedent to initiating operation of the fuel burner system,and to thereby insure that if any tube is defective so that it is unableto give an indication of flame failure, the system will be preventedfrom operating until a good tube has been substituted for the defectiveone. It is also desired to insure that the system will not functionunless tubes which are especially adapted to be used in a safety controlapparatus of this character are utilized in making tube replacements. Afurther object is to adapt the control apparatus for governing theoperation of a burner system having both pilot burners and main burners.For the purpose of attaining these and other objects the type ofapparatus illustrated in Figs. 3, 4 and 5 may be conveniently employed.

Referring first to Fig. 3, the control apparatus is there represented ascomprising a master unit MU which is operatively associated with aburner system including a pilot burner 50 and a main fuel burner 5|,these burners being supplied with fuel through separate supply pipes 52and 5 respectively.- The pilot burner 50 is so positioned with respectto the main burner 5| that a pilot flame originating in the burner 50may ignite fuel issuing from the nozzle of the main burner 5|. Aflame-sensing electrode I1 is mounted in association with these burnersin such a manner that it extends into the region of the pilot flame, andif gaseous fuel is being utilized in the main burner the electrode I]may also pass through the main burner flame zone. However, if atomizedliquid or pulverized solid fuel is being used in the main burner it maybe preferable to arrange the electrode l1 so that it barely skirts thefringe of the main flame as it enters the pilot flame zone, thusminimizing the possibility of fuel gathering on the electrode andforming carbon accumulations thereon, while at the same time insuringthat the pilot flame must be at least high enough to ignite the atomizedor pulverized fuel in order that it may contact the electrode. The pilotand main burner fuel supply pipes H and 53 are respectively providedwith fuel valves 54 and I! which are operated by solenoids I8 and 51,respectively. When these solenoids are in a deenergized condition thefuel valves 54 and II are closed so that fuel is not supplied to theburners 50 and ii.

The application of the control apparatus shown in Fig. 3 to a burnersystem having a plurality of main burners and pilot burners will beexplained subsequently in connection with Figs. 4 and 5, and for thepresent it will be assumed that the control apparatus comprises merely amaster unit MU controlling a single main burner 5i and associated pilotburner 50. Electrical current is supplied to the apparatus by a suitablesource of alternating-current voltage whenever the doublepole switch Sis closed. One of the poles of the switch 8 is connected directly by awire L0 to a terminal 58 on the control unit MU. The other pole of theswitch 8 is connected to a wire L! which serves as one of the line wiresfor the electrical system when the switch S is closed. The pole of theswitch S to which the wire Ll is connected is likewise electricallyconnected to another line wire Ll through the medium of a preliminarycontrol apparatus indicated by the rectangle designated 59, Fig. 3. Suchpreliminary control apparatus may comprise several forms ofcontact-making devices such as flow switches, time delay devices and thelike which all serve the function of insuring that the heater in whichthe products of combustion from the burner 5| are to be utilized isfirst thoroughly scavenged and otherwise conditioned for operationbefore any combustion can tak place. After the apparatus' 59 hasperformed this function it causes an electrical connection to beestablished between the line wire LI and the switch pole connecting withthe source of electrical current.

The line wires Li and L2 are respectively connected to terminals 60 and61 on the control unit MU, and conductors G2 and 53 lead from theseterminals to the primary winding of a transformer 64 included in theunit MU. The secondary of the transformer 6i supplies voltage to afilament 23 of the tube l8, this tube being similar to and performingthe same function as the tube I8, Fig. l. The tube 18 is provided with ashield grid that is directly connected to the cathode IQ of the tube,and the cathode is is also electrically connected to the center tap onthe secondary of the transformer 64.

An electron tube such as [8 of the character which we contemplateutilizing in the circuit of 13 Fig. 3 is customarily provided with eightprongs in its base, these prongs being adapted to be received incorrespondingly positioned contacts af forded in a tube socket as 9,Figs. 7 and 7A. The various elements of the tube are internallyconnected to respective prongs, and electrical connection between thesetube elements and the portions of the circuits external of the tube areestablished through the prongs of the tube and the contacts of the tubesocket in which these prongs are received, the various external circuitconductors being soldered or otherwise suitably secured to the aforesaidcontacts. In Fig. 3 the various tube socket contacts arediagrammatically represented as though viewed from the bottom and aredesignated I to 8, respectively. Ordinarily contact I is not used exceptwhen the tube is provided with an integral shield, in which event theshield is connected to the prong that fits into contact I, which is inturn grounded. However, in the present instance the electron tubes whichare to be employed in the apparatus are not shielded, or if they areshielded the shields are connected to ground in some manner other thanthrough the No. I contact in the tube socket. The filament 23is'connected to the prongs that are received in contacts 2 and I, acrosswhich the secondary of the filament transformer 81 is connected. Thecontacts numbered I, and 8 are respectively adapted to receive theprongs that are connected, to the anode 22, cont rol grid 28 and screengrid 65 of the tube I8.

For the purpose which will appear presently, we prefer to provide eachtube I8 with a special Jumper 66 which electrically interconnects theprongs of the tube I8 that are'adapted to be received in the contactsland 4 of the tube socket. If convenient, this jumper may beincorporated in the tube base in the course of manufacture thereof sothat the two prongs to which the ends of the jumper are connected willbe electrically in permanent connection with each other. The contact 4of the tube socket is connected by a conductor 61 to the conductor 62leading to the line terminal 60 of the control unit, while contact I isconnected by a conductor 68 to a tertube as I 8 shown in Fig. 3. Thisarrangement insures that the heater of the thermal switch II will not beenergized unless a tube particularly adapted for use in the control unitMU is employed. It should also be noted that the line wire LI, which isnot grounded, normally has a direct electrical connection with thecontact 4 of the tube I8 and thence through the jumper 66 to contact I,but if the jumper 66 is missing there is no electrical connection of'the line wire LI with the contact I. Under the latter conditions thecontact I is at substantially the same potential as the line wire L2,which is grounded as indicated at 86. (It will be understood, of course,that the wire L2 itself need not be grounded if the pole of the sourceof electrical current to which-this wire is connected by the switch S isgrounded.) Hence, if an operator should carelessly insert into the tubesocket a tube having a shield electrically connected to the prong whichfitsinto the No. 1 contact, but not being provided with a jumper 86,there is no danger that he will receive a shock, since the potential ofthe ungrounded or hot line wire LI is not applied to the shield of thetube.

The purpose of the thermal switch I3 is to afford a preliminarywarming-up period for the filament 23 of the tube It before any anodevoltage is supplied to the tube, this being a precautionary measuretoprevent damage to the tube. The heater II of the switch I! starts towarm up concurrently with the filament 23, and after .ample time haselapsed for the filament 23 to attain operating temperature, thebimetallic strip 80 ,ofthe switch It flexes and closes the switch toestablish an electrical connection between the a aforesaid conductor I2and another conductor ll minal 89 on the control unit MU. In the presentI instance, where only'a single control unit is being employed, theterminal 69 is connected by a Jumper 10 to a similar terminal II fromwhich a conductor I2 leads to one contact of a thermal switch generallydesignated I3. A branch conductor I4 electrically connects the conductor12 to a movable contact blade 15 of a relay I6 included in the controlunit MU. When the relay I8 is in a deenergized condition the blade I5engages a stationary contact 15a from which a conductor 11 leads to oneend of the winding I8 of-a heating element embodied in the thermalswitch I3, the other end of this heater winding being connected by aconductor I9 tothe conductor 63 which leads to the terminal BI on thecontrol unit MU.

When the line wires LI and L2 are potentialized a predetermined timefollowing closure of the switch S, a circuit is completed from wire LIthrough the conductors 62 and 61, jumper 86 in the base of the tube l8,conductor 68, jumper III, conductors 12 and H, relay contact 15a andconductor 11 to the heater winding I8 and thence by conductors I9 and 63back to the terminal 6| which is connected to the line wire L2. It is tobe noted, however, that this circuit could not be established if a tubewhich was not equipped with a jumper as 66 had been used in lieu of athat leads to one end of the winding of the relay 18,-the other end ofthis relay winding being connected by the conductor 82 to the conductorI9 leading to the line wire L2. Closure of the switchI3 therefore causesrelay It to energize, and-in so doing the relay moves its contact bladeI5 away from the contact 15a and Into engagement with a contact 15b. Theheater I! of the switch" is thereupon deenergized and the bimetallicstrip III thereafter cools off and separates the contacts of the switchII. The contact 15b of the relay I6 is connected to the conductor 8|which leads to the winding of this relay, so that immediately uponenergizatlon of relay 18 a holding circuit is established fromtheconductor I2 through conductor Hand contact 15b to the relay winding;hence opening of the switch II does not deenergize the relay I6, whichremainsiocked up through the just described holding circuit. Thus thethermal switch I3 merely performs the function of initially energizingthe relay I6 and is not called upon to operate continuously for holdingthe relay energized.

The relay I6 has another contact which is normally open when this relayis in a deenergized condition, but when the relay I8 is energized in themanner just described the contact 84 closes and electrically connects aconductor 83 with another conductor 85 that is tapped onto the cathodereturn wire 32. The conductor 83 is electrically connected by theconductors 61. and

' medium of the closed relay contact 84.

When the switch S is first closed, a circuit is completed through thewire L0 to the terminal the condenser 31 50, thence through a conductor90 to a stationary contact 96a, of the relay I6, thence through acontact blade 86. which is normally engaged with the contact 80a, and aconductor 81 to a terminal 83 on the control unit MU. An alarm 99, whichmay be of either the visible or audible type, is connected on one sideto the terminal 08 and on the other side to the line wire-L2, and hencewhen the switch S is first closed the alarm 89 is energized from thewire L to thereby indicate that the preliminary scavenging operationsreferred to hereinabove have commenced. When the relay I6 is energizedin the manner just described, however, there is a momentary interruptionin the operation of the alarm 89 while the contact blade 90 of the relaymoves out of engagement with the contact 09a and into engagement withthe contact 09b. A conductor 9I connects the contact 96b to a contact92a of another relay 93. A contact blade 92 is normally engaged with thecontact 92a when the relay 93 is deenerglzed, and this blade 92 isconnected by a conductor 95 to the cathode return wire 22 leading to thecathode l9 of the tube I9. Inasmuch as the wire 32 is potentialized bythe line wire LI upon energization oi the relay 19, as just explained.the alarm 09 is maintained energized from this source until such time asthe relay 93 becomes energized in a manner to be described.

The control grid 28 01 the tube It is connected through a protectivere'sistor29 and a conductor I00 to a terminal "II on the master unit MU,to which terminal the conductor 30 leading from the flame-sensingelectrode I1 is connected exteriorly of the control unit. The sheath 34enclosing the conductor 30 is electrically connected by a conductor IIto a terminal I02 on the control unit MU, while the resistor 3| andcondenser 91 (which perform functions analogous to those of thelikenumbered parts in Fig. 1) are connected in parallel between theterminals I02 and IN. The terminal I02 is connected by the filament of alamp 2! to a. terminal I03, and in the present instance where there isbut a single burner to be controlled. a Jumper I09 interconnects theterminal I03 with another terminal I05 to which the cathode return wire32 is connected. The pilot burner fuel pipe 52 is grounded as indicatedat I09 so that an electrical connection between the grounded line wireL2 and the electrode I1 is aflorded through the medium of a flame fromthe pilot burner 90 whenever such a flame is present. I

During the preliminary operations which have been described thus far,the fuel valves 94 and 59 remain closed and no flame exists in eitherthe pilot burner 50 or the main burner 5i; hence no current flow cantake place through the flamesensing circuit including the electrode I1and conductor 30. Inasmuch under these conditions, the control grid 28of the tube I9 is at substantially the same potential as the cathode I9.Anode voltage is supplied to the tube I9 through a path comprising aconductor I09 which leads from the or anode contact in the socket of thetube I0 to one side of the winding of a relay I09, this winding beingconnected on its other side by conductors H0 and III to the conductor 93leading to the terminal 0| to which the line wire L2 connected. It hasbeen previously explained that the cathode I9 and its return wire 32 areconnected to the line wire LI when the relay 16 is energized; henceenergization of this relay causes a voltage to be impressed across theanode lay these blades 22 and cathode I9 and thus produces a flow ofcurrent through the tube I9. This current flows only during forwardhalf-cycles of the alternating current, that is half-cycles in which theline wire L2 is positive with respect to the line wire LI, so that apulsating direct current is passed by the tube I8. The relay I09 is aslow-to-release relay which energizes when such a pulsating current issent through its winding and assumes a continuously operative conditionso long as such current flow continues.

The relay I09 has a pair of movable contact blades H2 and H3 which arenormally engaged with contacts IIZa and I'l3a when the relay isdeenergized, but upon energization of the rebecome separated from thecontacts I I2a and I I3a and engage the contacts I I2!) and II3b. Theblade H3 is connected by. a conductor Ill to the cathode return wire 32and the contact II3b is connected by a conductor H5 to a terminal IIS onthe control unit MU. For single-burner operation the terminal H9 isconnected by a jumper ill to a terminal II! on the control unit MU,which latter terminal is connected by a conductor II9 toanotherconductor I20 interiorly of the unit MU. The conductor I20 leads'to oneof a pair of spaced contacts I2I which are adapted to be bridged by theplunger of a push-button switch I22, the other of this pair of contactsI2 I being connected to a conductor I23. Normally the plunger of theswitch I22 is retracted into engagement with a pair of spaced contactsI24, one of which is connected to the conductor I23 and the other to aconductor I25, the conductors I23 and I25 being connected to terminalsI29 and I21, respectively, on the control unit MU. I

Assuming that the preliminary scavenging and like operations have beenduly performed and that the tube I9 has become conductive to energizethe relay I09, the operator manually depresses the push button I22 tothereby electrically connect the spaced contacts I2I. Thereupon acircuit is completed from the cathode return wire 5 32 (which isconnected to the line wire LI) through conductor I, contact I I36 ofrelay I09, conductor II5, jumper II'I, conductor H9, contacts I2! andconductor I23 to the winding 01' relay 93, thence through this relaywinding and conductors I28 and II I to the line wire L2. Re-

lay 93 is thus energized and closes a normally open contact I29 thereof,at the same time disengaging the contact blade 92 from the contact 92aand engaging this blade with the contact 92b 5 that is connected to theaforesaid conductor I20.

No. 2 05 r) Disengagernent of the blade 92 and contact 92a breaks thecircuitto the alarm 09 and causes this alarm to cease operating.

Engagement of the blade 92 of the relay 93 as no charge xist on o withthe contact 92b thereof closes a circuit from the cathode return wire 32through conductor 95, contacts 92 and 92b, conductor I20, contacts HI 0!the switch I22 (provided the plunger of this switch is maintaineddepressed by i the operaand conductor I23 to terminal I29 and thenceexteriorly of the control unit MU through a conductor I30 to the windingof the solenoid 56 which operates the pilot fuel valve 54, this solenoidbeing connected on its other side to the line wire L2 to complete thecircuit. A parallel circuit is likewise established under control of thswitch I22, this circuit extending from the conductor I23 through aconductor I3I connected therewith, contact blade H2 and contact II2b ofrelay I09 (this relay still being in an energized condition) r 17 and aconductor I92 to a terminal I99 on the control unit MU. and thence thecircuit continues exteriorly of the unit MU through a conductor III tothe primary of an ignition transformer I95 and. from this primarywinding back to the line wire L2. Thus the pilot fuel valve solenoid 99and theignition transformer I95v are concurrently energized when thepush button I22 is de- Pressed. The pilot burner 50 is equipped with aspark plug I96, the terminal of which is connected by a conductor I31'to one end of the secondary winding of the transformer III, the otherend of this secondary, being grounded. High potential is thus applied tothe spark plug as the pilot fuel valve opens so that the fuel issuingfrom the 5 pilot b""urner*l0'ma'"ybe ignited.

and II2b and reengage the contacts H21; and

I Ila, respectively, one of the results of this being that the circuitto the primary of the ignition 'Upon the establishment of a pilot flamein the burner "electrical current commences to flow through theflame-sensingcircuit', but due to the rectifying property of the pilotflame such current will flowbetween the electrodes I1 and the burner 99only when the electrode I1 is positive with respect to ground potential.In other words,.

current flow through the pilot flame takes place only during thosehalf-cycles when the line wire LI is positive with respect to the linewire'L2. The flame-sensing circuit is traced from the line wire LI tothe cathode return wire 92 in the man- I ner described hereinabove,thence, through the jumper I04, lamp 92, resistor 9| and, condenser,

91 in parallel (as well as through the dielectric insulating mediumbetween the sheath I9 and conductor 90), flame-sensing electrode I1, andthence through the pilot name to the pilot burner 90. which iselectrically at the same potential as the line wire L2. However, duringthe half-cycle in which such current flow may take place through theflame-sensing circuit the tube I9 is e inoperative inasmuch as thecathode I9 which is connected to the wire 92 is then positive withrespect to the anode 22, which is connected through the winding of therelay I09 to the conductor III leading to the line wire L2. That is tosay, the current flow through the flame takes place only during theinverse, half-cycles of the alternating current, and therefore arectifled current flows through the condenser 91 as well as through thecapacitance between the sheath 34 and conductor 90, the direction ofthis current flow being such as to cause the terminal I92 of the controlunit MU to assume a potential which is positive with respect to that ofthe terminal IN. the lamp 99, Jumper I04 and wire 92 to the cathode I9of the tube I9, while the terminal IOI is connected through theconductor I00 and protective resistor 29 to the control grid 29 of thistube; therefore, the grid 29 becomes negative with respect to thecathode I9 due to the unidirectional charge which accumulates on thecondenser 31. v

The charge on the condenser 91 (which will hereinafter be assumed toinclude the capacity of the insulation between the conductor 90 andsheath 94) may continue to build up throughout a plurality of inversehalf-cycles until a substantially steady-state condition is attained solong as a flame is present in the burner 90; During forward half-cyclesof the alternating cur-.

rent, when the relativepolarities of the line wires LI and L2 are suchthat the anode 22 of the tube I9 is positive with respect to "thecathode Terminal I02 is connected through I transformer I95 is brokenand the spark plug I99 is deenergized. Also, the electrical connectionbetween the conductors H4 and H5 is broken so that the contact I2I ofthe switch I22' whichis connected to the conductor I20 is no longerconnected to the cathode return wire 92 through the path including theconductors -I It, II! and Ill. However, when the relay 99 was energizedin the manner hereinabove explained, a circuit was closed from the wire92 through conductor 95, contacts 92 and 92b of the relay 99, andconductor I20 to the lust-described contact I2I of the switch I22.Hence, so long as the relay 93 remains energized the energizing circuitto the pilot fuel valve solenoid .06 continues, providing the pushbutton I22 is held depressed.

The relay 99 may be maintained energized through two possible holdingcircuits, one of these being traced from the cathode return wire 92through the aforesaid contacts 92 and 92b of the relay 99, conductorI20, contacts I2I of the switch I22 and conductor I29 .to the winding ofthe relay 93. The other holding circuit is established when the contactblade H9 of the relay I09 engages the contact Illa upon deenergizationof this relay, this circuit being traced from the cathode return wire 92through the conductor I4, con'acts III and la, a conductor 0, contactI29 of the relay 93 (this contact being closed so long as the relay 9!is energized, conductor I29. winding of relay 99 and conductors I29 andIII to the line wire L2. The relay 9! remains locked up through thislatter holding circuit so long as relay I09 remains deenergized. Duringthe interim while the blade III of the relay I09 is traversing the gap,between the spacedv contacts H911 and Ilia upon deenergization of therelay I09, the relay 93 is held ener-' gized solely through theflrst-mentioned holding circuit including the contacts 92 and 92b of therelay 99 and the contacts I2I of the push-button switch I22, and hencethere is no interruption ofthe circuit through which the pilot fuelvalve solenoid -59 is held energized. The same result couldbe obtainedwithout the use of this manually controlled holding circuit by arrangingthe contacts 9:1 and H911 of the relay I09 in a r'nake-before-breakmanner so that the blade II! engages the contact II9a prior todisengaging the contact I I9b. However, in the present instance it ispreferred not to employ make-before-break relay contacts but instead toutilize the manually controlled holding circuit for the aforesaidpurpose.

To summarize the foregoing operations, the establishment of a pilotflame in the burner causes the tube I 9 to cease conducting current andbrings about deenergization of the relay I09, as an incident to which aholding circuit is established for maintaining the relay 93 energized solong as relay I09 remains deenergized. As can HI and the contact M tothe conductor |4| leading to a lamp I42 and thence through the filamentof this lamp and conductors I43 and I II to the line wire L2. The lampI42 thereupon lights to indicate that a holding circuit has beenestablished by the relay I09 for the relay 93 and the pilot fuel valvesolenoid 56. Upon observing this indication the operator releases thepush button I22, and the plunger of the switch is thereupon restored tobreak the electrical connection between the contacts |2I and reestablishthe connection between the contacts I24. This results in automaticallyclosing a circuit from the conductor I23 through the contacts I24 andconductor I25 to the terminal I21 and thence through the conductor I45and the winding of the solenoid 51, which control the main fuel valve55, back to the line wire L2. The solenoid 51 energizes and opens thevalve 55 to admit fuel to the main burner I, and as the fuel issues fromthe burner 5| it is ignited by the pilot flame from the burner 50 andthus the operation of the heater is initiated. As an incident to this alamp I46, which is connected across the conductors I25 and III, lightsto thereby indicate that the main burner 5| is in operation.

If for any reason there should be a failure of the pilot flame, or ifthis flame becomes too low or is otherwise rendered ineffective toignite the fuel discharged from the nozzle of the main burner 5|, sothat there is an interruption of the electrical connection through theflame between the electrode I1 and the pilot burner 50, rectifiedcurrent ceases to flow in the flame-sensing circuit and the condenser 31thereupon loses its charge. The potential of the control grid 28 of thetube I8 is thus brought to approximately the same level as that of thecathode I9, and the tube IB is thereupon rendered effective to passcurrent during the forward half-cycles. This results in energization ofthe relay I09, as an incident to which the lamp I42 is extinguished andthe holding circuit for the relay 93 is broken, causing this relay torelease and also bringing about the deenergization of the pilot valvesolenoid 55, main valve solenoid 51 and lamp I46. As the relay 83 isdeenergized, the blade 92 thereof engages the contact 92a and therebycloses circuit to the alarm 89, whereupon the alarm operates to givewarning of what has happened.

The valves 54 and 55 close when the solenoids 56 and 51 are deenergizedto thereby interrupt the admission of fuel to the burners 50 and 5|. Inorder to start the burners in operation again, it is necessary to repeatthe above described steps commencing with actuation of the startingswitch I22 and culminating in the opening of the main fuel valve 55. Ifcombustion conditions in the heater are favorable so that the fuelissuing from the burners 50 and 5| is properly ignited, the systemoperates automatically to continue the supply of fuel to these burnersunless and until another abnormal condition arises.

It is to be noted from the foregoing that in the enabling the relay 93and pilot valve solenoid 56 as well a the ignition transformer I25 tobecome energized when the push button I22 is depressed, after which thetube |8 becomes nonconductive upon the establishment of a pilot flameand deenergizes the relay 1 I09 to enable the main valve solenoid 51 tobecome energized when the push button I22 is released. Thus, if there isan internal failure of the tube l8 which causes this tube to bepermanently nonconductive or substantially so, the pilot fuel valve 54is prevented from opening. On the other hand if the tube I3 fails insuch a manner that it is permanently conductive, it will be possible toopen the pilot fuel valve 54 but the main fuel valve will be preventedfrom opening because of the fact that the relay I09 cannot bedeenergized. In either event the operator is duly apprised of thesituation from an observation of either the burners themselves or theindicating lamps I42 and I46, and he may then shut down the systementirely by opening the switch S to enable a. check to be made.

As in the case of the previously described embodiments of the invention,it is desirable to check the system for abnormal conditions other thanfailure of combustion which may be a source of danger, and particularlyto automatically detect any defective conditions in the flame-sensingcircuit. Thus, for example, the electrode I1 may become deformed so thatit engages the burner 50 or 5| and is thus connected directly to ground,that is to say, to the line wire L2. This causes the grid 26 to bepositive with respect to the cathode I 8 during forward half-cycles, andhence the tube passes current to energize the relay I09 and therebyinterrupt operation of the burners 50 and 5| in the manner hereinaboveexplained. Similarly, if a non-rectifying or bilateral leakageresistance is established between the electrode l1 or conductor 30 andground, such as may be caused by carbonization of the electrode I1 orfailure of the insulation around the conductor 30, this nullifies thecurrent-rectifying action of the flame and causes an alternating currentto pass through the condenser 31. On alternating current the condenser31 acts more or less as a shunt and is not likely to build up anyappreciably steady charge; or in other words, any charge which isimparted to this condenser during an inverse half-cycle may be lostduring the succeeding forward half-cycle. This results in rendering thesystem inoperative in the same manprocess of initiating operation of themain burner 5| the tube I8 must initially be in a conductive conditionfor energizing the relay I09 and thereby ner as though there had been aflame failure or ground-out condition, the fuel supply either beinginterrupted immediately or being prepared for interruption in the eventof a flame failure, depending upon the value of the leakage resistance.Likewise, failure of the insulation between the conductor 30 and thesheath 34 so as to establish a low-resistance leakage path in shunt withthe condenser 31 renders this condenser ineifective to bias the controlgrid 28, with the above described results.

If more than one set of pilot and main burners is included in the burnersystem it is advantageous to have the fuel supply to all of the burn--as controlled by a single master unit MU, the operation of which is inturn partially controlled by one or more secondary units SU, Fig. 4, themaster unit MU being allocated to one of the main burners 5| and itsassociated pilot burner 50 and each secondary unit SU being likewiseallocated to a respective main burner 5| and pilot burner 50. Thesecondary units SU are of identical construction with respect to eachother but are greatly simplified as compared with the master unit MU.Thus, referring to Fig. 4, each secondary unit SU includes a tube I8which is identical with the tube I8 of the master unit MU, Fig. 3, andwhich functions in a corresponding manner. Each unit SU may include anindividual filament transformer 64', the primary of which is connectedacross the line wires LI and L2, but it is to be understood that asingle transformer 64, Fig. 3, may be utilized to supply current forheating all of the tube filaments. Each tube I8 is provided with ajumper 66' which is electrically connected to the prongs that arereceived in the tube socket contacts I and 4 as in the case of the tubeI 8, Fig. 3.

When the master unit MU is to be operated in conjunction with one ormore secondary units SU the jumper I between the terminals 69 and II,Fig. 3, is omittedand a conductor I50, Fig. 4, leads from the terminalII of the master unit 'MU to the terminal 68' on the last secondary unitSU in the series. A conductor I5I connects the terminal 68 of the masterunit MU to the terminal II of the first secondary unit SU in the series.Interiorly of this secondary unit a conductor I52 leads to the No. 4tube socket contact, and if a tube I8 properly equipped with a jumper66' is received in the tube socket the circuit continues from theconductor I52 and the No. 4 contact through this jumper 66 to the No. Icontact and thence through a conductor I53 to the terminal 69 on thesecondary unit SU. A conductor I54 then leads from this terminal 69 tothe terminal I I on the next secondary unit SU in the series or if thereis only one other secondary unit SU, this conductor I54 is connected tothe terminal II of such unit. If there are more than two secondary unitsSU the terminal 69' of each such unit is connected by a conductor as I54to the terminal II on the next unit, except for the last unit SU in theseries,

the terminal 69' of which is connected to the conductor I50 leading backto the terminal II, Figs. 3 and 4, .of the master unit MU. As a resultof these connections, if the proper typ of tube I8 has been installed ineach secondary unit SU, a series circuit is established through thejumpers 66 and 66 of the tubes I8 and I8 in the master and secondaryunits, as can be seen by reference to the simplified circuit diagram inFig. 5. This series circuit connects the line wire LI with the conductorI2 leading to the time delay device generally designated I55 in Fig. 5,

this device I55 comprising the thermal switch I3 and relay I6, Fig. 3,which are wired up in the manner shown in the latter view. The circuitthen continues through the heater element and I relay winding of thedevice I55 and through a conductor I9 back to the line wire L2.

When the conductors LI and L2 are potentialized, assuming that theaforesaid series circuit has been established, the filaments of thetubes I8 and I8 immediately commence to heat up during the time delayinterval which is afforded by the action of the device I55, Fig. 5. Atthe expiration of this interval the device I55 closes its contact 84,thereby connecting the cathode return wire 32 to the conductor 61 whichis connected to the line wire LI, and thus applying the potential of theline wire LI to the cathode return wire 32. The device I55 likewiseshifts its contact blade 86 into engagement with the contact 86b tothereby close a, circuit from the wire 32 through the alarm 89 to theline wire L2 for 22 maintaining the alarm 89 energized so long as therelay 93 remains deenergized.

Each secondary unit SU has a relay I58, the winding of which isconnected on one side by a conductor I59 to the No. 3 or anode contactof the tube I8 in said unit, Fig. 4, and on its other side by aconductor I60 to the terminal BI which is electrically connected to theline wire L2, Thus, the relay I58 is included in the anode circuit ofthe tube I8 to be energized whenever the tube I8 is rendered conductive.Terminals I05 of all the secondary units SU are electrically connectedto the cathode return wires 32' of their tubes I8 and are electricallyconnected exteriorly of the secondary units SU to a common conductor Iwhich leads back to the terminal I05 of the master unit MU, Fig. 3.Thus, referring to Fig. 5, the cathodes of the tubes I8 and I8 aresimultaneously potentialized when the relay device I55 connects thecathode return wire 32 of the master unit MU with the line wire LI Thecontrol grids 28 of the tube I8 in the various secondary control unitsSU, Fig. 4, are respectively connected to the flame-sensing electrodesI'I associated with the pilot burners 50 to which the units SU arerespectively allocated. When a pilot flame is present in a particularburner 50 a circuit is established from the line wire L2, Fig. 5,through said burner 50 and the path afforded by the pilot flame to theassociated electrode II, thence through the conductors 30 and I00 andthe resistor3l, condenser 31 and lamp 33', Figs. 4 and 5, included inthat particular control unit SU to the cathode return wire 32', fromwhich point the circuit is completed through the common conductor I65and wire 32 in the master unit MU to the line wire LI. Rectified currentflowing through such circuit builds up a charge on the condenser 31'included therein so that the associated tube I8 is biased to the pointwhere it is nonconductive. However, when a flame is absent in theparticular pilot burner 50 the tube I8 is rendered conductive and passescurrent through its anode circuit to energize the relay I58.

Referring again to Figs. 3 and 4, the relay I09 in the anode circuit ofthe tube I8 in the master unit MU has a contact 31) which is connectedto a terminal II6 on the master unit MU. For multiple burner operationthe jumper I [1, Fig. 3, is omitted and a conductor I56, Fig. 4, isemployed to connect this terminal I IE to a terminal IS! on the first ofthe secondary units SU in the series. The relay I58 in this secondaryunit has a contact I6l which is normally open when the relay isdeenergized but which closes when the relay is energized, in a mannerpresently to be explained, to electrically connect the terminal I51 onthe secondary unit SU to a terminal I62 on said unit. The terminal IE2is connected by a conductor I63 to the terminal as IE1 of the nextsecondary unit SU, and the terminal as I62 of this unit is in turnconnected by another conductor I63 to the terminal as I51 of the nextsucceeding secondary unit as SU, and so on until the last of thesecondary units SU in the series is reached. The terminal I62 of thislast unit is connected by a conductor I66 to the terminal I I8 of themaster unit MU, Figs. 3 and 4, Thus, referring to Fig. 5, all of therelays I 09 and I58 in the various control units must be energized inorder to establish an electrical connection between the conductors H4and H9, Figs. 3 and 5. Before operation of any of the burners can beinitiated the tubes I8 and I8 must all be ren- Y 23 dered conductive,and if the tubes are all in working condition they will automaticallybecome conductive as soon as the cathode return wires 82 and I2 arepotentialized inasmuch as there is no substantial bias on the controlgrids of these tubes in the absence of flames in the associated burners.Assuming that all of the tubes are in good condition so that the relaysI" and Ill in their anode circuits become energized, a series circuit iscompleted from the cathode return wire 82 of the master unit MU. Fi 3,through the conductor Ill and the contact Illb of the relay I" andthence through the contacts Iii of the relays I" to the conductor I",Figs. 4 and 5,1eadin: back to the conductor I It, Figs. 3 and 5, whichis connected to the conductor I20 leading to a contact III of the switchm, The system is thereupon conditioned to become operative when thebutton of the switch I2! is pushed.

As has been described hereinabove, the switch III is manually actuatedto energize the pilot fuel valve solenoid It and, the-ignition means. Inthe present instance, where a plurality of pilot burners are to beignited, it is advantageous to employ a distributor Ill, Fig. 5, havinga number of contact segments I'll, each of which is connected by aconductor ill to a spark plug I" on a pilot burner It or II. Thedistributor III has a wiper IIl insulatedly mounted on the shaft I" of amotor Ill. The motor I'll is connected in parallel with the primary ofthe ignition transformer I", while the high potential terminal of thetransformer secondary is electrically connected by a conductor I10 tothe wiper I'll of the distributor Ill, The motor "I is energized andcommences rotating the wiper I'll simultaneously with the energisationof the ignition transformer I" and the opening of the pilot fuel valveN. As the wiper I'll successively enages the contacts "I, the sparkplugs I" are energized in rapid succession for igniting the fuel in thevarious pilot burners I and 50', By employing such a distributorarrangement, it is possible to utilize a single ignition transformerwhich need only be capable of energizing a single spark plug at a time,and on the whole a more satisfactory and economical operation is insuredthan would be the case if a separate ignition transformer were providedfor each burner or if a single transformer were arranged to supplypotential to a number of spark plugs in parallel.

As each pilot burner 50 is ignited a rectified current flow isestablished in the flame-sensing circuit associated with such pilotburner, and in the manner explained hereinabove, this causes thecorresponding tube I8 to become nonconductive, thereby causing the relayin in the anode circuit of such tube to deenergize. In the presentinstance, however, it is desirable that the relay I08, Figs. 3 and 5, inthe anode circuit of the tube II of the master unit MU remain energizedat least until all of the tubes I8 in th secondary units BU have beenrendered operative in response to the establishment of pilot flames intheir corresponding burners 50', regardless of whether the flame in thepilot burner 50 associated with the master unit MU has been established,for so to do insures that the holding circuit for the relay ll and pilotvalve solenoid 56 will not be estab lished, nor will the ignitiontransformer I35 and motor I" be deenergized, at least until flames arepresent in all of the pilot burners M.

For the purpose of delaying the response of the master unit MU to theestablishment of a pilot flame in the burner 50 until the last of thePilot flames in the burners 50' has come into existence, the jumper IIMbetween the terminals I03 and III! of the master unit MU, Fig. 3, isomitted and the terminal I03 is connected by a conductor I'II to aterminal Ill! in the first of the series of secondary units EU. Theterminal ill on this unit BU is electrically connected by a contact I19of the relay I58 to another terminal III on this unit whenever the relayI58 is in a deenergized condition, but when the relay is energized thisconnection is broken. The terminal I" of the secondary unit BU isconnected by a conductor Ill to the terminal I18 on the next secondaryunit EU in the series until the last of the secondary units SU isreached. The terminal I" of this last unit is connected by a Jumper I82to the terminal ID! to which the common conductor III is connected.

Thus,referring to Fig. 5, an electrical connection between the terminalsIll and III! of the master unit MU, Fig. 3, is not established until allof the relays III in the secondary units BU, 8. 4, have been deenergizedto close their'contacts Ill; that is tosay, the flame-sensing circuit ofthe master unit MU does not become effective until flames have beenestablished in all of the pilot burners ISO to render all the tubes IIin the secondary units 8U nonconductive. Thereupon the flame-sensingcircuit of the master unit MU is rendered effective, and if a flame ispresent in the pilot burner 50 the tube ll becomes nonconductive todeenergize the relay I". The relay I" closes its contact Ilia andthereby establishes a holding circuit for the relay II and the pilotfuel valve solenoid 58, at the same time opening its contact I Ilb todisable the ignition means including the transformer I 35 and motor Ill.The lamp I42 flashes on as the relay I09 closes its contact II2a toindicate that the push button I22 may be released to initiate operationof the main burners BI and I in the manner previously explained.

If an abnormal condition should arise at any of the burners with whichthe secondary units 8U, Fig. 4, are associated, such as a failure ofcombustion in such burner, th -tube II in the secondary unit SU becomesconductive and energizes its relay I58. As this relay energizes it opensits contact I19 and thereby breaks the flame-sensing circuit for themaster unit MU. This causes the tube I! in the master unit MU to passcurrent and thereby energize the relay I08, causing the relay contactIlla to open and break the holding circuit for the relay 93, which inturn results in deenergizing the fuel valve solenoids 58 and 51, thusinterrupting operation of all of the burners in this system. If theabnormal condition should arise at the burner with which the master unitMU is associated, then the tube I8 is immediately rendered conductivewith the effect just described. It is manifest that the apparatus willfunction in a safe manner for any of the abnormal conditions referred tohereinabove, such as ground-out or leakage effects, as well as for afailure of combustion. It should also be noted that if any of the cablesheaths 34 or 34 becomes grounded, the associated warning lamp 33 or 33'lights to indicate the location of the fault.

While the master unit MU and secondary units SU have been describedhereinabove as constituting separate control units which may be mountedin individual cabinets that are wired together by external connectiomand independently located with respect to each other, it is to beunderstood that these various control units may be embodied mmmmhxgipallghp geries circuit exthkiipg through the jumpers.

trically connected to the cathode return wire 32 i by way or a seriescircuit extendin through the contacts I19 of the relays I58 in thesecondary units SU, whereas the condensers 91 and resistors II of thesecondary units SU are con-'' prises a relay unit BU whichcontainscertain of the relays and associated devices for controllingoperation or the fuel supply and ignition means.

Operation 01 this relay unit RU is] in turn governed by theflame-detecting units 0; olieoi'y which is operatively associatedwiflijeach pilot" 1 burner 00 in the system. The controlLunits U an electrontube I92 and new; III-havingits winding connected in the anode circuit"or said tube. The filaments oi the tubes'ylll are pretalarm89 back tothe line wire L2. Operation of the alarm 99 thus signifies that thenecessary preliminary operations have been performed and that the heatersystem is ready for operatlon.

' As the line wire LI becomes potentialized the voltage existing betweenthe wires LI and L2 is impressed across the cathodes I91 and anodes I99of the various tubes I92. The control grids .205 of the tubes I92 arerespectively connected byv conductors 209 to terminals 201 on thecontrol units U.'irom which terminals the conductors 90 lead totherespective flame-sensing electrodes I1 associated with the pilotburners 50. The

sheaths 94 o! the conductors 30 are respectively.

connected toterminals 299 which are electrically connected through theiillments of lamps 209 to the terminals I99 on the units U. Each controlunit U has a condenser 2I9 and a resistor 2I I which are electricallyconnected in parallel across the terminals 201 and 208. Since there isno J. current flow through the various name-sensing outfits-includingthe electrodes I1, conductors are 01 unii'orm construction and eachincludes 7 erably supplied with current by individual illamenttransformers I94 having their primaries,

connected across terminals I99 and. llllwhich are respec vely connectedto the linewires L2 and L0. e cathodes I91 01 "the tubes I92 areconnected to terminals I99 which are electrically connected to the linewire LI. The anode I99 of each tube I92 is connected by a conductor 200to one end of the-winding 01 the relay I99, the other end ofthia'..wlnding being connected by the conductor 29!" to the terminalI95. It will be understood that although the relay unit EU andflame-detecting units U are represented in Fig. 8 as constitutingseparate units, they may be conveniently incorporated in the samecabinet to afford a single control unit for the entire burner system. i

It will be recalled that the line wire LI ls'not immediatelypotentialized upon closure of the switch S but must await theestablishment of 90,.conde'nsers 2I0andresistors 2 prior to theestablishment orpuot 'flames in the burners 90.

the relay unit RU to initiate operation of the an electrical connectionwith the wire L0 through the medium of the preliminary control apparatus99. Hence, while closure of the switch S immediately potentializes thewires L0 and. L2 for thereby energizing the filament transformers I94and initiating heating of the cathodes I91, the line potential is notimpressed upon the anode circuits of the tubes I92 until the scavengingand other preliminary operations have been performed under control ofthe apparatus. 59. Ordlnarily this afiordssufllcient time for the tubesI92 to be conditionedior operation, but if a longer delay period isdesired this can be afforded by an arrangement such "as that comprisingthe thermal switch 19 and relay 19, Fig. 3. It is likewise apparent thatthe series circuit arrangement oi Jumpers 99 an 69', Figs. 4 and 5, maybe employed to insure that the control apparatus is not conditioned foroperation unless tubes of the proper type are installed in all of theactive control units U. At the completion of the aforesaid scavengingcycle the line wire LI assumes the same potential as the line wire L0,whereupon a circuit is established from wire LI through a conductor 202,a contact blade 203 of a relay 204 included in the relay unit RU, acontact 203a with which the blade 203 cooperates when the relay 2 isdeenergized, and thence thro g an pilot burners 90. Each relay contact2I2 is arranged-to'bridge a pair of terminals 2I3 and 2 on itscontrolunit U when the relay I93 is energized. The terminal 2 of thelast control unit U in the series is connected to the line wire LI,while the terminal 2 oi each of the other units U is connected by aconductor M5 to the terminal 2I3 of the next succeeding unit U in theseries. A conductor 2; leads from the terminal 2|! 0! the first unit Uto a contact 2031) 01 the relay 204 in the unit RU, Irom which relaycontact the blade 203 is normally disengaged when the relay 204 isdeenergized. A push button 2I1 is arranged to bridge a pair of spacedcontacts 2I9, one of which is connected to the conductor 2 and the otherto a conductor 2I9 leading to the winding of the pilot fuel valvesolenoid 59. 'Assuming that all of the tubes I92 areconducting currentso that all of the relays I99 are energized to maintain their contacts2I2 closed, an electrical path is established from the line wire LIthrough the relay contacts 2I2 in series to the conductor 2 I6. However,if any of the tubes I92 is defective so that its relay I93 remainsdeenergized, the conductor 2IB remains disconnected from the line wirLI.

Assuming that all of the tubes I92 are conducting current, operation ofthe pilot burners 50 is initiated by manually closing the switch 2I1

